In some situations there is a desire to monitor flow of fluid downwell, for instance the inflow in an oil or gas production well at a given depth/distance into a well. The rate of flow of product at the top of the well can be relatively easily be determined, for example by using a suitable flow meter. However the inflow at different depths/distances into the well may be of interest. For example it may be wished to determine the relative contribution to the total flow of the various different sections of the well which provide an inlet for the oil or gas.
This may be useful for long term monitoring and/or to provide useful information for planning future wells. Typically the formation of a production well involves drilling into a rock structure which holds a reservoir of hydrocarbons and performing a perforation step, where shaped charges are fired to perforate the rock and provide a flow path for the oil/gas. Typically there may be many perforation sites at different distances into the well. Monitoring the flow, in subsequent operation, from each perforation site may provide useful information for planning the perforation sites in other wells. Also, in formation of some wells there may also be a fracturing step following perforation, e.g. hydraulic fracturing where a fluid is forced into the well under pressure, to fracture the rock to release the oil/gas from the rock and provide a flow path. Monitoring the flow for the various in-flow sites may provide information about how successful the fracturing step has been and whether the flow is coming from all sites evenly or whether there are significant differences in flow at different parts of the reservoir. Monitoring the flow may also provide indications about changes in the flow from different parts of the reservoir over time.
Also in some instances a well may be divided into a number of different production zones which are effectively owned or leased by different organisations. Thus there may be a need to determine the relative contribution to the total flow from each production zone.
It may also be desired to monitor out-flow in injection wells, for example to monitor that the injected fluid is being injected into the reservoir evenly.
The use of permanent flow meters at different depths within a well is not generally practical due to the difficulties in providing suitably rugged equipment that can survive the harsh conditions in a production well for long periods of time, and the difficulties in installing such equipment with a suitable power supply and means of relaying the flow data to the surface. Typically therefore flow readings are acquired periodically by inserting wire line tools having one or more flow meters into the well on a temporary basis and taking flow readings at different depths. However the use of wire line tools involves halting normal well operation and is a relatively expensive procedure.
Various fibre optic sensors have been proposed for use downwell. Fibre optic sensors interrogate an optical fibre and analyse the backscattered radiation, either from deliberate point sensors within the fibre (e.g. Fibre Bragg gratings or the like) for from the intrinsic scattering sites within the fibre itself, to determine various parameters such as strain, vibration or temperature.
For example, fibre optic distributed acoustic sensing (DAS) is a known technique whereby a length of optical fibre is optically interrogated, usually by one or more input pulses, to provide sensing of acoustic activity along its length. Optical pulses are launched into the fibre and the radiation backscattered from within the fibre is detected and analysed. By analysing the radiation backscattered within the fibre, the fibre can effectively be divided into a plurality of discrete sensing portions which may be (but do not have to be) contiguous. Within each discrete sensing portion mechanical disturbances of the fibre, for instance, strains due to incident acoustic waves, cause a variation in the properties of the radiation which is backscattered from that portion. This variation can be detected and analysed and used to give a measure of the intensity of disturbance of the fibre at that sensing portion. Fibre optic distributed temperature sensing is also known and again relies on optically interrogating an optical fibre and analysing backscattered radiation. By analysing the backscattered radiation over time temperature changes at various parts of the optical fibre can be determined.
The use of fibre optic sensors downwell can be advantageous as the fibre optic cable can be made relatively rugged and thus can survive in a well environment and no power is needed downwell. The nature of the sensor means that data is readily acquired from different distances into the well.
WO2010/136773 teaches using such acoustic data to monitor various activities related to well formation and operation and suggests that DAS may be used for flow monitoring. This document teaches that the optical fibre to be used for sensing may be included in the well during the stages of well formation and that the optical fibre may be attached to the outside of an outer casing forced into the well bore which is subsequently cemented in place. This provides good acoustic coupling for the fibre and doesn't interfere with subsequent well operation. It also means that the fibre may be used for sensing during subsequent steps in well formation such a perforation.
However there are a significant number of existing wells in which no fibre is present.
In addition the use of optical fibre on the outside of the casing does typically mean that the optical fibre will be present when the perforation charges are fired. Typically, after the wellbore has been drilled, casing is inserted throughout substantially the whole of the working length/depth of the well and then cemented in place (for at least part of the well)—usually by forcing cement through the casing to the bottom and out to back fill the void between the casing and wellbore. This occurs before perforation. During perforation care should therefore be taken to orient the perforation charge away from the fibre to avoid severing the fibre when the charges are filed. The exact orientation of the perforation charges and position of the fibre are not generally known and so techniques such as magnetic anomaly detection may be used which add to the complexity and expense of well formation.
It would therefore be desirable to be able to monitor production flow at different depths within a well in use without the need for inserting any special instruments into the flow. It would also be desirable to be able to monitor flow in existing wells.